MOLLUSCA 



PELECYPODS 



227 



Exogyra (Fig. 96). Upper Jurassic-Cretaceous. 



Attached like Ostrea by the left valve but distinguished by the 



strongly twisted beak of this valve. Right or upper valve 



usually fiat. Surface varying from nearly smooth to strongly 



A 



B 



Fig. g6. — The oyster-like pelecypod, Exogyra arietina Roemer, from the Del Rio 

 clay of the Washita formation (uppermost Comanchean) of Texas. A, right valve 

 shown in place within the larger left valve. (After Hill.) B, outer view of a slightly 

 imperfect left valve. C, inner view of same; ad., adductor muscle scar; /., liga- 

 ment groove. Natural size. 



plicate. The name refers to the twisted beaks from the Greek 

 cxo, out, + gyros, a curve. 



^ij^^r:'-i 



^ J* 



k^^^ 



"•^^^^.l^jC. 



Fig. 97. — An oyster-like bed formed by Exogyra arietina in the ancient seas 

 covering much of Texas during uppermost Comanchean (Del Rio) time. See 

 Fig. 96. (X i) (From Hill.; 



